One type of thermal printer used for printing of high quality images employs a thermal head assembly comprising a multitude of closely spaced resistance heating elements. These elements are separately energized in accordance with image data to transfer very small dots of ink or pixels from a thermally reactive dye element onto a print element or page. The image data is in the form of binary words (e.g., 8-bit words) which define each of the positions and densities of the respective pixels and the image thus printed. To produce a high quality printed image with fidelity to an original, the printed pixels should be very small and have a nearly uniform tone scale over a wide range of densities. A thermal print head for such a printer can, by way of example, be a linear, one-dimensional array of thick (or thin) film resistance-heating elements individually formed on a ceramic substrate which is then mounted on a heat sink, such as a metal bar. At present, print heads with 300 miniature resistance elements per lateral inch (2400 elements for an 8 inch wide page) are commonly used. These elements are small enough to give a good degree of visual sharpness. However, minor variations in printing action of these individual resistance elements give rise to visually objectionable non-uniformity (for supposed equal density) in the pixels on a page being printed. This non-uniformity manifests itself in visible lines, streaks and bands in a direction parallel to page motion past the thermal head.
Circuits to compensate electronically for print non-uniformity due to minor differences in the individual resistance elements in a thermal printer have been developed and are currently in use. In U.S. Pat. No. 4,827,279, which is assigned to an assignee in common with the present patent application, there are described a method and apparatus for correcting non-uniformity in the printing action amongst the resistance elements of a thermal print head. In the compensating system disclosed in this patent, correction numbers for each resistance heating element of the head assembly are derived from microdensitometer measurements of a line of print and the correction numbers are then computed according to a specified formula. These correction numbers, which are unique to a particular head assembly, are then entered in a high speed electronic memory circuit of the overall thermal printer apparatus. The numbers so stored are used along with input image print data during subsequent normal operation of the printer to achieve improved quality of printing.
However, in addition to variations in the resistance heating elements themselves, variations in the energy applied to each of them by a power supply in the thermal printer can also cause non-uniformity in printing. The human eye is extremely sensitive to even minor differences in gray scale in a printed image. Thus, relatively small changes in power supply voltage can result in visually objectionable variations in printed image density where supposedly equal density is expected. The current drawn from the power supply in a thermal printer such as described above changes in magnitude by thousands of times between a condition in which only one print element is energized and a condition in which all of the elements are simultaneously energized. It is possible to substantially eliminate changes in power supply voltage under these normal operating conditions of the printer by providing a highly regulated power supply. This approach entails a large and expensive power supply and adds considerably to the size and cost of the overall printer apparatus. Moreover, a regulated supply does not eliminate small voltage variations at the resistance elements themselves due to voltage drops in supply busses from the power supply to the resistance elements. Various attempts have been made in the past to electronically compensate for changes in supply voltage to the resistance elements in a thermal printer. However, these attempts have not been as successful as is desired, or else they were relatively expensive, or both.
It is desirable to provide a highly efficient and accurate system of compensating for supply voltage variations, as well as variations in overall print head temperature, in a thermal printer to achieve substantially uniform print density (where a given density is called for). This system should be fully compatible with existing digital control and head drive circuits (such as described in the above-identified U.S. Pat. No. 4,827,279) and be highly cost effective.